It is known to provide extrusion dies in which the extrusion orifice from which a strand of thermoplastified synthetic resin emerges with a pair of lips at least one of which can be adjustable, i.e. displaceable toward and away from the other lip to define the gap width between the lips and hence the orifice width and the thickness of the resulting extruded strand.
It is important in the operation of an extrusion press, e.g. a worm system for thermoplastifying the synthetic resin and which is provided at its output side with the extrusion die, to be able to adjust the gap width with high precision and to maintain an adjusted gap width with narrow tolerances and so that it is controllable with very brief setting times and practically no dead time in the setting response. The precision requirements for such systems have grown with the development of plastic technology.
In the extruder die with respect to which the present invention constitutes an improvement (see U.S. Pat. No. 2,938,231), the positioning rod is a metal bolt or pin whose head is formed with a screw thread and which is threaded into a corresponding bore in the die body. The shaft of this bolt is heated by ohmic heat via a respective heating element and the thermal expansion of the bolt or rod generates an adjustment displacement or throw of the device in response to the control and/or regulating unit which provides fine control or regulation of the displacement of the lip and hence of the thickness of the extruded strand which is thus produced.
The coarse setting is effected by screwing the bolt further into the respective bore utilizing the thread formed on the head and cannot be effected in a tolerance-free manner. It is also possible to effect coarse adjustment by adjusting the counterlip. The coarse adjustment and the fine adjustment can be coupled together in this manner so that the tolerances of the coarse adjustment can affect the precision of the fine adjustment.
The heating element is associated with the control and regulating system and allows the bolt shaft to be set at an adjustable temperature within a predetermined temperature range and to be maintained at this temperature until control or regulation requires another increment of displacement, whereupon the temperature of the bolt shaft is changed.
The drawbacks of such a system should be apparent. On the one hand, a temperature increase in the bolt shaft can be developed only slowly for physical reasons so that the setting time for establishing a particular precision of the lip is long and the system has substantial adjustment inertia or dead time, i.e. a slow response. On the other hand, because of practically unavoidable temperature inhomogeneities of the bolt shaft, a precision of the control or regulation, once the die has been warmed up, cannot be ensured because of thermal coupling in the system.
In the system of German patent document 34 27 912, and U.S. Pat. No. 4,592,710, which operates, by comparison, with much higher precision and much smaller setting time and dead times, i.e. with faster response, the positioning elements are field translators.
Field translators are piezoelectric effectors or magneto-strictive effectors, i.e. electrically controllable positioning elements operating in accordance with the piezoelectric effect or the magnetostrictive effect. While such systems have been found to be successful for many purposes, such field translators are not capable of generating extremely high positioning forces and are also not capable of maintaining such forces for long periods.